Increased productivity and high out-of-coal seam dilution (25% to 30%) in the US and around the globe continue to generate dust control problems in mining areas. After a significant decrease in the number of incidents of coal worker's pneumoconiosis (CWP) over the last several decades, the number of reported cases in this decade is increasing. The primary cause of CWP is inhalation of respirable dust in a confined workplace; specifically, the inhalation of coal and quartz dust in a mine. The National Institute for Occupational Safety and Health recognizes this disease as being severely disabling, potentially lethal, and entirely preventable through respirable (less than 10 micron) dust control. The typical protocol for prevention of this disease has been monitoring mine workers for symptoms of this disease and, once a CWP diagnosis has been made, moving the miner to a low-dust exposure job. Prevention of this disease through a significant reduction in mine workers' exposure to respirable dust is a high priority. Additionally, several mines are now facing reduced dust standards due to high respirable quartz content in the dust. In underground US coal mines, miner operator (MO), haulage unit operator (HO), and roof bolting (RB) unit operator are typically overexposed to respirable dust.
The conventional approach to dust control in a mine has been the use of water sprays located on the mining machines to wet the coal. Approximately located and intuitively designed water sprays on the cutter drum and around the continuous miner chassis have been extensively used to control dust for the miner operator (MO), batch haulage unit operator (HO), haulage roadways, and material transfer points. A continuous miner or CM is extensively used for coal production in partial extraction mining areas. Typical spray systems, provided by manufacturers, have 15-45 sprays located across the top and the sides of the cutter boom (FIGS. 1A and 1B). In addition, under-the-boom and loading pan sprays on some miners provide water sprays to contain and wet the dust in the face area. However, there is no consensus in the art area on the type and location of sprays, volume of water and water pressure to be used in sprays. Although general guidelines have been developed by researchers based on laboratory and field studies, there is no systematic method of design or apparatus for using a spray system to meet the specific conditions to be encountered.
Several studies over the last several decades have attempted to locate the source of and have attempted a solution to the dust problems in mining environments. The conventional wisdom is that presented by Chang and Zukovich (Cheng L and Zukovich P. P. 1973. Respirable dust adhering to run-of-face bituminous coals. Pittsburgh, Pa.: U.S. Department of the Interior, Bureau of Mines, RI 7765. NTIS No. PB 221-883.) Their position was that a large amount of dust created does not become airborne and stays attached to the broken material. Therefore, spraying more water on the broken material tends to reduce dust. Adding water directly at the cutting picks that gets mixed with fragmented coal is more important than creating a shroud of water around the miner or shearer. Based on this observation, the conventional practice of mixing the water uniformly with broken coal was developed. However, this approach alone has not been effective in mine dust control.
More recently it has been observed that water can be used to control dust through the wetting of broken material and capture of airborne dust. (Kissel, F., “Handbook for Dust Control in Mining”, NIOSH, Information Circulation (IC 9465), 2003, pp. 131.) Although the methods of wetting broken material have been more uniform throughout the industry, a haphazard approach has been taken to the capture of airborne dust through the use of water sprays. This is most likely due to the problem and sometimes conflicting proposed solutions. It is suggested that a large number of smaller-volume sprays is better for dust control than smaller number of larger-volume sprays. Jayaraman and others concluded that many spray systems can create turbulent airflow in the face area that can result in rollback of dust. (Jayaraman, N, Fred N. Kissel, and W. E. Schroder (1984), “Modify Spray Heads to Reduce Dust Rollback on Miners,” Coal Age, June 1984)
However, certain research has proven valuable in the design of water spray systems. Courtney and Cheng concluded that typical water sprays operating at 100 psi do not capture more than 30% airborne dust in an open environment. (Courtney W. G. & Cheng L. 1977. Control of respirable dust by improved water sprays. In: Respirable Dust Control—Proceedings of Technology Transfer Seminars, Pittsburgh, Pa., and St. Louis, Mo., IC 8753, pp. 92-108. NTIS No. PB 272 910.) Furthermore, inappropriately designed sprays can displace dust clouds rather than wet or capture airborne dust. Reducing the water droplet size through the use of atomizing or fogging sprays may temporarily improve the airborne dust capture efficiency. However, small droplets tend to collapse/evaporate easily and release the captured dust. (McCoy J., Melcher J., Valentine J., Monaghan D., Muldoon T. & Kelly J. 1983. Evaluation of charged water sprays for dust control. Waltham, Mass.: Foster-Miller, Inc. U.S. Bureau of Mines contract no. H0212012. NTIS No. PB83-210476.) Atomizing nozzles are most efficient in airborne dust capture followed by hollow cone, full cone, and flat sprays. Hollow cone sprays are less likely to clog due to larger orifice area.
Nozzles operating at higher pressures are likely more efficient in the use of water while providing similar airborne dust capture efficiency as those operating at lower pressures. However, high-pressure sprays tend to disperse more dust. Therefore, their use is more appropriate in a relatively confined environment.
Courtney and others reported that the primary release point for dust from a CM is from under the boom when the cutter head shears down. (Courtney, W. G, N. I. Jayaraman, and P. Behum (1978), “Effect of Water Sprays for Respirable Dust Suppression with Research Continuous Mining Machine”, BuMines RI-8283, 17 pp) Thus, under-boom sprays should be considered. However, location and maintenance of under-boom sprays presents significant problems. Jankowski reported results for an alternate under-boom spray system with about 25% improved dust reduction (Jankowski, Robert A, N. I. Jayaraman, and C. A. Babbitt (1987), “Water Spray System for Reducing Quartz Dust Exposure of the continuous Miner Operator,” Proceedings of the 3rd U.S. Mine Ventilation Symposium, Pennsylvania State University, PP 605-611.)
In spite of considerable excellent research by the U.S. Bureau of mines (USBM) and the National Institute of Occupational Safety and Health (NIOSH) over the last 40 years, there are significant limitations to the current practice. These include use of high water pressure on the chassis (100 psi or more); similar water pressure on the chassis and under-boom sprays leading to escape of airborne dust from the sides; only one point of dust control on the top of the chassis; no control on roll-back dust travel; use of only one type of sprays such as hollow-cone for all sprays; poor orientation of sprays, etc. There is a need to revisit the design concepts of sprays on continuous miners to control respirable dust (including quartz dust) in and around the mining face area.
In the industry there is a need for improving spray efficiency. A more reasoned and systematic design is needed that more effectively reduces the respirable dust around mining machinery.